1. Field of the Invention
The present invention relates to high speed optical digital transmission systems in general, and more particularly, the present invention relates to an optical photoreceiver in a fiberoptic digital transmission system.
2. Description of the Related Art
The transmission of speech, data, video and other information using the visible and infrared portion of the electromagnetic spectrum is commonly known as optical communication. A basic optical communication system is illustrated in FIG. 1. Information is transmitted from an information source 100 to an information user 102 using an optical transmitter 104, an optical channel 106, and an optical receiver 108. The most common light source used as an optical transmitter 104 generally includes a light emitting diode, a laser diode or a laser and modulator pair. The optical channel 106, which refers to a transmission path between the optical transmitter 104 and the optical receiver 108, is a glass fiber made of silicon dioxide. Some optical fibers are also made of transparent plastic. Finally, the optical receiver 108 is generally a semiconductor photodiode, with the two most commonly used semiconductor photodiodes being the p-i-n photodiode and the avalanche photodiode.
When analog signals, such as voice, are transmitted digitally, the transmission rate, or bit rate is dependent upon both a rate at which the analog signal is sampled and a coding scheme that is used. The analog signal can be accurately transmitted if the signal is sampled at a rate of at least twice the highest frequency contained in that signal. For example: since most of the energy in normal speech is contained in frequencies below 4 kHz, standard telephone channels need only transmit messages with frequencies up to 4 kHz. Therefore, the standard 4 kHz telephone channel is sampled 8000 times a second, and since a decoding procedure uses 8 bits to describe the amplitude of each sample, a total of 64,000 bits/second are transmitted for a single telephone message.
Random fluctuations in a received signal are commonly referred to as xe2x80x9cnoisexe2x80x9d. One of the problems associated pith optical receivers in telecommunications systems involves the existence of noise at the photoreceiver side. For example, when a low level of light is detected directly in a photodiode, the electrical signal level generated by the photodetector is too low, or is small compared to thermal noise at an output of the photodetector. As a result, the signal is lost in the noise. For a typical p-i-n photodiode, this loss limits the sensitivity of the photodiode to xe2x88x9230 dBm of optical power at the input in order to maintain a reasonable bit error rate at 1 Giga-bit per second (Gbps). Therefore, in order to increase the data transmission rate above 1 Gbps, it is necessary to amplify the signal before the signal reaches the photodetection stage.
Although many attempts have been made, increasing the transmission rate in conventional telecommunication systems has proven to be problematic. For example, an avalanche photodiode, which has been used to increase the transmission rate, is limited to a net gain-to-noise ratio of approximately 10, which limits the avalanche photodiode to a transmission rate of approximately 10 Gbps. As a result, in order to extend the transmission rate to 100 Gbps using the avalanche or pin photodiodes, it is necessary to include optical preamplification. Conventional use of optical preamplification relies on post detection amplification, or amplification of the signal after the signal has been detected, which tends to be very difficult. To give a specific example, since significant low frequency information is contained in the data, it is necessary to have an electrical amplifier positioned after the photodetector that is both broadband (10 GHz bandwidth, for example) and that has a low frequency cut-off in the 10 kHz range. Because of the multiple decade difference that exists between the two ranges, constructing an electrical amplifier having those properties has been difficult. As a result, increasing the data transmission rate above 10 Gbps has proven to be a very demanding and difficult task.
A conventional optical receiver is illustrated in FIG. 2. An incoming intensity modulated light signal is detected and converted to an electrical pulse stream, or electrical signal by a photoreceiver 20, such as a p-i-n photodiode or an avalanche photodiode. The electrical signal is amplified by a low-noise preamplifier 30 and a linear amplifier 34. The signal is further amplified and leveled by a limiting amplifier 32. The electrical signal from the limiting amplifier 32 is input to a retiming stage 22 that includes a clock recovery circuit 24 and a retiming circuit. or flip flop 26. After receiving the electrical signal, the retiming stage 22 outputs a retimed electrical signal.
As a result of standards which have been developed for a synchronous optical network, commonly referred to as the xe2x80x9cSONETxe2x80x9d standards, a received optical power level of xe2x88x9230 dBm (1 micro W) is required at a data rate of, for example, 10 Gbps. Therefore, the photoreceiver output electrical signal is usually in a range of tens of microvolts. Typically, the highest frequency for which the sensitivity of a photoreceiver utilizing a p-i-n photodiode is better than xe2x88x9230 dBm, is approximately 1 Gbps. As the frequency of transmission is increased to 10 Gbps, the power level necessarily increases to approximately xe2x88x9220 dBm, and to xe2x88x9210 dBm when the transmission rate is increased to 100 Gbps, due to the thermal noise at the output of the photoreceiver.
The electrical signal from the photoreceiver 20 is usually below the required output level needed by the clock recovery 22 and the flip flop 24. Thus, once the digital signal is converted to an electrical signal, the electrical signal is amplified in an electrical amplification stage 28 prior to being input to the retiming stage 22. The electrical amplification stage 28 includes a low noise preamplifier 30, a limiting amplifier 32, and in some cases a linear amplifier 34. The amount of amplification is dependent upon the input optical signal level, the conversion factor of the photoreceiver 20, and the signal level required for the limiting amplifier 32.
The preamplifier 30 of the amplification stage 28 is a fixed gain low noise amplifier. The limiting amplifier 32 amplifies the signal from the preamplifier 30 and linear amplifier 34, with variable gain, to adjust the gain to a fixed level. The signal is further processed in order to recover the timing in the retiming stage 22 using the clock recover circuit 24. The recovered clock is input to the flip flop 26 and used to reshape and retime the amplified digital data stream to account for pulse distortion and broadening so that the integrity of the data is preserved after many, possibly thousands of retiming/regenerating operations. The electrical amplification stage 28 provides a fixed output electrical signal, typically in the range of 1 volt peak-to- peak, which is needed by the clock recovery circuit 24 and flip flop 26 to reliably extract information.
Since light is attenuated as it travels in a fiber, the electrical gain required in conventional intensity modulated digital fiber optic systems is usually high. As described above, typical photoreceiver optical input sensitivities are xe2x88x9230 dBm at 1 Gbps for a bit error ratio of 1.0xc3x9710xe2x88x929 with a p-i-n photodiode integrated with a low noise preamplifier, and xe2x88x9236 dBm for the same system operating with an avalanche photodiode. With an input of one microwatt (xe2x88x9230 dBm), the photocurrent produced can be as high as 1 xcexcA through 50 ohms. Therefore, the electrical power out of the photodetector is xe2x88x9276 dBm. A typical flip flop requires at least 100 mV (xe2x88x9216 dBm) to operate reliably, which yields a minimum of 60 dB electrical gain. The gain increases to 80 dB when 1 volt is needed for the flip flop. In addition, it is desirable for the photodetector to work when the optical input signal is as high as xe2x88x9210 dBm, which would yield an optical system with a 20 dB dynamic range (xe2x88x9210 to xe2x88x9230 dBm). When the optical power increases to xe2x88x9210 dBm, the electrical gain must decrease by 40 dB for the output power from the limiting amplifier to remain constant. As a result, the operating gain range of the limiting amplifier must be twice that of the optical dynamic range.
At a transmission rate of 100 Gbps or more, it is difficult to obtain an input sensitivity of xe2x88x9230 dBm using the typical prior art electronic receiver of FIG. 2 without the addition of some sort of optical gain or avalanche gain in the photoreceiver prior to the electrical amplification. But, as described above, the avalanche photodiode is limited to approximately 10 Gbps. Furthermore, as bit rates increase above 1 Gbps, the minimum optical input signal level must also increase so that the same bit-error-ratio is maintained. This requirement is a fundamental limitation of p-i-n photodiodes operating at low light levels because the electrical signal level generated by the photodiode is very close to the thermal noise floor of the input electronic preamplifier, and therefore, as the bandwidth of the system increases, the bandwidth of the noise in the preamplifier must also increase, and therefore the input optical signal must increase. As a result, using conventional optical receivers to obtain a transmission rate of 100 Gbps or more is problematic.
It is therefore, an object of the present invention to provide a high speed fiber-optic digital receiver for information at an operating rate from 1 Megabit per second (Mbps) to over 100 Gbps.
It is a further object of the present invention to provide a high speed fiber-optic digital transmission system that obtains an amplitude limited output voltage greater than one volt directly from a photodetector without addition of linear or gain-control electrical amplification stages.
Additional objects and advantages of the present invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Objects of the invention are achieved by an apparatus that includes a first optical amplifier which amplifies an optical signal, and a high current photodetector which converts the amplified optical signal into an electrical signal.
Further objects of the invention are achieved by an optical receiver for receiving a signal from an optical fiber in a fiber-optic digital transmission system. The optical receiver includes a first optical amplifier which amplifies the signal, a high current photodetector which converts the amplified signal to an electrical signal, and a retiming circuit which is driven by the electrical signal. The first optical amplifier amplifies the signal so that the electrical signal from the high current photodetector has a power level sufficient to drive the retiming circuit without requiring further amplifications
Further objects of the invention are achieved by an optical receiver for receiving an optical signal from an optical fiber in a fiber-optic digital transmission system that includes a first optical amplifier to amplify the optical signal. The first optical amplifier has an input 3 dB compression point lower than the desired input sensitivity at a desired bit rate. A second optical amplifier further amplifies the amplified optical signal. The second optical amplifier has an automatic gain control for output signal leveling at a desired operation point of the transmission system. A high current photodetector converts the further amplified optical signal from the second optical amplifier to an electrical signal that drives a retiming circuit. The electrical signal from the high current photodetector has a power level sufficient to drive the decision circuit without requiring further amplification, and output signal levels of the high current photodetector are approximately equal to 1 volt peak-to-peak, 0.5 volts peak-to-peak, and 0.25 volts peak-to-peak for systems with data rates of 1 to 10 Gbps, 10 to 40 Gbps, and 40 to 100 Gbps, respectively.